The invention is, for example, concerned with the type of container described in GB-A-1424680. In this reference, an envelope with a zipped opening is described. At one end of the zipped opening is mounted the lock unit, this lock unit having a raised housing or chamber with an opening on one side. The fastener tab of the zip has a central aperture so that it can fit over the chamber and then a disposable sealing element (known for simplicity as a “seal”) is pushed into the chamber to lock the fastener tab in place and thus lock the zipped opening. The seal is a substantially flat element, moulded from a suitable plastics material, which has a head part and a body part. Between these two parts is a weakened or frangible section and it is the head part which locks into the chamber by means of the resilient barbs of an arrowhead configuration. The dimensions of the seal are such that the weakened or frangible section is at the mouth of the chamber. The seal cannot then be removed whole from the chamber and must be broken in order to release the fastener tab of the zip. The lock unit and disposable seal thus provide tamper evidence to the container. Such a tamper evident container (zipped envelope) is shown in FIG. 6, labelled “prior art”.
For the avoidance of any doubt, while the invention is concerned with a zipped envelope of the type discussed above, the invention which is set out below is also applicable to other types of containers where one part (lid, flap etc) can cooperate with another part (the main body of the container, for example) which has a lock unit.